Download DATABASE VIEWS

DATABASE VIEWS
CHAPTER 5 (6/E)
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CHAPTER 8 (5/E)
LECTURE OUTLINE
 Database views provide convenient usage
• Virtual view realized when query uses that view
 Materialized views allow efficient re-use
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• Must be recalculated or updated when base tables change
VIEWS FOR CUSTOMIZATION
 Consider database(s) describing university’s activities
• Academic institution
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Students, professors, classes
Grades, transcripts
Admissions, convocations
Alumni
• Corporate institution
• Finances, human resources
• Board of Governors
• Capital assets
• Charitable institution
• Donors, fundraising activities
• Research institution
• Granting agencies, industrial/non-profits/academic partners
• Grants and contracts, intellectual property, licensing
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 Each user group provided appropriate “subset” of the data
• e.g., some financial/scheduling info relevant to most groups;
other info confidential
• Underlying data shared, not silo’d.
 Updates must be seen by all affected users.
VIEWS (VIRTUAL TABLES)
 Consider again the query
SELECT title, year, genre
FROM Film
WHERE director = 'Steven Spielberg' AND year > 1990;
• Returns all matching films currently in the database
• If re-run after updates, will give revised table of matches
 A view is an unexecuted query that can be run on demand.
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• Single table derived from other table(s)
• A virtual table
USING VIEWS IN SQL
 CREATE VIEW command
• View name and a query to specify the contents of the view
CREATE VIEW Big_Earners AS
SELECT E.Ssn AS Ssn, E.Lname AS Name,
E.Salary AS Salary, M.Lname AS Manager
FROM EMPLOYEE E, EMPLOYEE M
WHERE E.Super_ssn = M.Ssn and E.Salary > M.Salary;
 Queries can use view as if it were a base table.
SELECT *
FROM Big_Earners
WHERE Salary < 100000;
 View always up-to-date
• (Re-)evaluated whenever a query uses the view
• Keeping it up-to-date is responsibility of the DBMS and not the user
 DROP VIEW command
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• Dispose of a view
UPDATING A VIEW
 What if an update is applied to a view as if it were a base table?
CREATE VIEW Big_Earners AS
SELECT E.Ssn AS Ssn, E.Lname AS Name,
E.Salary AS Salary, M.Lname AS Manager
FROM EMPLOYEE E, EMPLOYEE M
WHERE E.Super_ssn = M.Ssn and E.Salary > M.Salary;
UPDATE Big_Earners
SET Salary = 100000
WHERE Name = 'Smith';
• Change corresponding tuple(s) in base table(s)
• Tuple might disappear from view!
• WITH CHECK OPTION clause at end of view definition ensures new
and updated tuples match view definition (else error)
• Deleting tuple from view might require update to base table instead of
deletion from base table
• e.g., deletion from CS338 view ≟ deletion from UW database?
• What if Salary defined as sum of two base attributes or as aggregate
such as SUM or AVG?
• What if Big_Earners defined as a UNION of two tables?
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• Not all views are updateable.
MATERIALIZED VIEWS
 If the base tables don’t change, neither does the view instance.
• Re-executing view definition each time view is used is wasteful if
base data has not been updated.
 Solution: view materialization
• Create a temporary view table when the view is first queried
• Keep view table on the assumption that more queries using the
view will follow
• Use materialized view (if it exists) to answer future query
• Requires efficient strategy for automatically updating view table
when the base tables are updated
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Options when any base table is updated:
1. Delete the materialized view
2. Rematerialize the view
3. Incrementally update the view
• DBMS determines what new tuples must be inserted, deleted,
or modified in materialized view
LECTURE SUMMARY
 Views are virtual or derived tables.
 Can be used for any query wherever base table can appear
 May or may not be updatable
• Unions, joins, and (aggregate) functions are problematic
 Materialized views used to save query time
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• Must be kept up-to-date if base table(s) updated